(1) Field of the Invention
The invention relates to a method of and a device for operating a gas discharge lamp which is fed with an alternating voltage or an alternating current, the instantaneous power of the lamp being increased (pulsed mode of operation) at given time intervals. The invention also relates to apparatus equipped with such lamps and devices as well as to methods of treating an electrode which are based on said mode of operation.
(2) Description of Related Art including information disclosed under 37 CFR 1.97 and 1.98
Such modes of operation and devices are known, for example, from WO 96/174724 or from U.S. Pat. No. 5,608,294. The cited WO publication discloses a device with an energy supply circuit for operating a gas discharge lamp wherein the energy supply circuit provides an alternating voltage or an alternating current of predetermined period duration in order to feed the gas discharge lamp with a predetermined power in such a manner that, when the mean lamp power is reduced relative to the nominal power, the instantaneous power is increased within one half period duration directly prior to the pole reversal of the alternating voltage or the alternating current. This brief increase of the instantaneous power prior to the pole reversal ensures that the re-ignition voltage required after the pole reversal essentially need not be increased relative to the voltage in the nominal mode of operation.
The cited United States patent describes a method of operating a gas discharge lamp with a short light arc; therein, the lamp receives an alternating current of a given period duration and a brief current pulse is superposed on the lamp current in each half period, which current pulse has the same polarity as the lamp current in the relevant half period so that the constancy of the light arc and the durability of the electrodes of the gas discharge lamp are essentially improved.
The variation of the current intensity or the voltage, as known from the cited publications and referred to hereinafter as xe2x80x9cpulsed operationxe2x80x9d or xe2x80x9cpulsed modexe2x80x9d, has proven to be very effective in practice. It is to be noted that in this context the terms xe2x80x9cpulsed operationxe2x80x9d and xe2x80x9cpulsed modexe2x80x9d are to be understood to cover all variations of the current strength or voltage in time where additional current or voltage pulses are superposed on the operating current or the operating voltage, notably for the purpose of stabilization of the lamp arc (in many publications (for example, see EP 0 865 210 A2, WO 97/247871 or U.S. Pat. No. 5,428,408), however, the term xe2x80x9cpulsed operationxe2x80x9d is to be understood to mean exclusively a lamp mode of operation in which the lamp operates in quickly repeated, very short periods of time and does not output light during a large part of the time).
Although pulsed operation can considerably improve the constancy of the light arc, the service life is not yet satisfactory; this is important notably in the case of high-pressure gas discharge lamps with a very short light arc, such as used, for example in data and video projectors with LC or mirror displays (deformable mirror device), but also for various other applications. The shorter the light arc required, the more severe the effects of burning off of the lamp electrodes and the accompanying extension of the light arc between the electrodes will be. It is not seldom that burning off of the lamp electrodes in gas discharge lamps with short and very short light arcs during the first 100 hours of operation already reduces the efficiency in, for example, a projection system, by 20%.
Moreover, the manufacture of gas discharge lamps with a very short electrode gap is extremely difficult, since the electrodes are normally sealed in a quartz tube and are positioned, prior to the sealing into the tube, in such a manner that, due to the manufacturing process, their position deviates from the original setting after the finishing of the lamp, that is, both in respect of the gap as well as in respect of the lateral alignment with one another. The positioning tolerances of the electrodes can be reduced at great expense only.
A further problem that can be solved with great difficulty only is posed by the geometrical shape of the electrodes themselves. Granted, it is possible to cut desired electrode geometries from a solid material, but for reasons of cost that the electrodes preferably consist of an electrode rod (drawn tungsten wire) with a tungsten spiral slid thereon, even though the geometry and the inner structure of the electrodes, ultimately defining the heat distribution, can be controlled to a lesser extent in such a construction. In lamps having a short light arc the enormous thermal loading of the electrodes already causes fast transport of the electrode material (for example, evaporation of tungsten) which, within a few hours, can completely change the electrode front face in high-pressure gas discharge lamps having an arc length of approximately 1 mm. Even an ideally shaped electrode will usually retain its original functional properties for only less than 100 hours in such circumstances.
Considering the foregoing, it is an object of the invention to provide a method and a device for operating a gas discharge lamp which make it possible to use the transport processes taking place during operation of a gas discharge lamp advantageously for the formation of the electrodes.
The object is achieved by a method in which processes inherent in gas discharge lamps improve formation of the electrodes. In particular, the lamp can be operated in a mode which produces useful light, while at the same time the lamp electrodes are rejuvenated by growth of structures on the electrode tips which compensate for bum-off of electrode material and reduce the arc length to the desired value. The magnitude of the structures which grow on the electrodes is proportional to the operating frequency, while the diameter is smaller as operating frequency is raised.
The object is achieved notably by means of a method of the kind set forth wherein the values of at least one operational datum of the lamp which varies in time are continuously or discontinuously measured and the frequency of the alternating voltage or the alternating current (operating frequency) is selected in dependence on the measured values. The operating frequency is then advantageously selected in dependence on the measured values of at least one operational datum from the group of operational data which includes the overall service life of the lamp, the operating voltage, the power taken up or given off, the arc length and the electrode gap, since all such data offer direct or indirect information concerning the state of the electrodes, notably the electrode gap (for example, even in the case of a new lamp having a service life, an idea of the approximate state of the electrodes, and hence of the necessity of selecting a given operating frequency, can be derived from the service life itself on the basis of experimental values).
Those of ordinary skill will recognize that, as used herein, operating frequency refers to a frequency used after starting power for ignition and lamp warm-up have been provided at start-up frequencies, and that increasing the power of the lamp at a succession of given time intervals is a repeated action different from any changing of the lamp power which occurs during warm-up. Also, starting frequency, at the beginning of an electrode regeneration operation, is not related to start-up frequencies used during ignition and warm-up.
The invention is based on the novel insight that the magnitude of the structures growing on the electrodes during operation with alternating current or alternating voltage is proportional to the operating frequency of the current or the voltage. It has been found that the diameter of the structures grown is smaller as the fundamental frequency of the operating current or the operating voltage is higher. Typical frequencies in high-pressure gas discharge lamps lie between approximately 40 and 600 Hz. For lamps of a given type (for example, in conformity with DE 38 13 421 A), for example, the following relation holds: approximate diameter of the structures grown=a/f1/2, where f is the operating frequency in Hertz and a is a lamp-specific proportionality constant which lies typically between approximately 2000 and 5000 pm Hz1/2, so that structures having a diameter of from approximately 200 to 500 xcexcm are formed in the case of a fundamental frequency of 100 Hz. Generally speaking, this constant may lie in a slot between 1000 and 10,000 xcexcm Hz1/2. The height of the structures formed generally is smaller than their diameter and as a rule amounts to approximately from 0.4 to 0.8 times the diameter. The ratio, however, has been found to vary between 0.2 and 1.2. This relationship is used according to the invention so as to form projecting electrode tips in a controllable manner during operation of the lamp.
The invention enables formation of the electrode during operation, independently (within given limits) of the basic shape of the electrodes as imposed by the manufacturing process. The desired electrode gap, or the desired operating voltage, can be adjusted within given limits by utilizing the transport processes. When the desired voltage is reached, the conditioning process is interrupted and the lamp is operated at the frequency prevailing at that instant.
It is a special advantage of the method according to the invention that it can be applied time and again during the service life of the lamp, thus allowing to some extent a xe2x80x9cregenerationxe2x80x9d of the electrodes so that outstanding results can be achieved over a very long service life.
Because of the physical laws of the transport processes, the electrode structures formed during operation are situated practically exactly opposite one another, so that no lateral offset occurs. When the operation commences with sufficiently low frequencies, the structure will be situated at the electrode center.
To this end, the measured values are advantageously monitored in respect of the satisfying of predetermined secondary conditions and when a first secondary condition is satisfied (start condition), the lamp is operated with a low operating frequency (starting frequency) until a second secondary condition is satisfied, after which the operating frequency is increased. Such start conditions may be, for example, the putting into operation of a new lamp for the first time or the building up of the necessary operating voltage beyond a given limit value. It is notably also possible to define different start conditions with different starting frequencies so that, for example, successive structures of decreasing diameter can be built up on the electrodes when a new lamp is put into operation for the first time, a start being made with a comparatively low starting frequency, whereas in the case of electrodes whose shape is to be modified only slightly it may suffice to build up small structures immediately and to start with a comparatively high starting frequency.
The operating frequency can be continuously increased for successively building up the structures. However, it has been found that it is particularly advantageous to increase the operating frequency in discrete steps until a predetermined interruption condition is reached. Such interruption conditions may be: the reaching of a predetermined operating frequency (maximum frequency), the reaching of a predetermined minimum operating voltage, constancy of the electrode gap over a predetermined period of time.
A device which is arranged to achieve the described object in operating a gas discharge lamp is provided with measuring means for the continuous or discontinuous measurement of the values of at least one operational datum of the lamp which varies in time and with means for changing the frequency of the alternating voltage or the alternating current (operating frequency) in dependence on the measured values. Such a device can also be simply used for or be provided in already manufactured gas discharge lamps and lighting devices used for gas discharge lamps of all types, notably projectors, lighting systems of trucks etc.
A preferred embodiment of the device includes a compact evaluation and control unit which includes at least one microprocessor and is intended to control the operating frequency, the operating voltage and the alternating current applied to the gas discharge lamp, as well as to evaluate and monitor the measured values in respect of the satisfying of predetermined or selectable secondary conditions; advantageous use can then be made of processors and units already provided in existing devices for the pulsed operation of gas discharge lamps.